In contemporary computer systems, magnetic tape storage is used primarily for mass storage of data, usually for archival purposes. For those purposes, random access is not required on a frequent basis. Thus, typically, a steady stream of data is recorded sequentially on the tape medium.
As the density of data storage on magnetic tape has been increased both by increasing the number of data tracks written on a given size of tape (track density) and by increasing the amount of data placed on each track, checking of the data written has become important. Adapted for such checking is a configuration of the read/write transducer or head having a "core" or "gap" for writing followed immediately (from the point of view of the data written on the tape) with a read gap to check data as it is written. The read gap typically is narrower than the write gap in such a situation so as to avoid reading adjacent tracks which are in close proximity in high density recording and which might otherwise be read if there is lateral tape motion.
Currently, the read and write cores are among the most expensive components of a magnetic tape device. Thus, tape drives for recording on multiple longitudinal data tracks, typically 50-500 tracks, do not employ 50-500 cores, one for each track. Instead, such multiple track tape drives may employ only one or two write cores to record, respectively, one or two tracks at a time as the tape moves in one direction. When the end of the tape is reached, the direction of tape motion is reversed, the tape head assembly is shifted transverse to the direction of tape motion, and another one or two tracks are written. This reversal and shifting is repeated until recording is completed or the tape is filled. The additional time required relative to fewer sweeps of a head assembly with more write cores generally is tolerable for archival applications.
As mentioned above, newer tape drives have achieved increasingly higher data densities by increasing track density using more and narrower tracks, for example, up to 500 tracks on 1/2-inch magnetic tape. These newer drives utilize narrow read gaps that can read the wider tracks written by older tape drives and thus allow for so-called "forward compatibility." However, there is also need for "backward compatibility," in which the newer drives can record for reading by older tape drives with wider read gaps.